Originally posted by Williams Audio hi,
who can help, i need schematics for counterpoint pwr amps sa-100/sa-220 . it is hard to find this stuff, so any help will be very much appreciated.
best regards
williams

Hi Williams Audio,
1. What is wrong with your unit? I am trying to help you.

2. The SA-220 is very close to the SA-100 in general concept. All the ss stuff on the output board is an analog computer designed to sense the actual junction temperature of the outputs. It is jumpered to it's least effective position (basically off) and has no effect on the circuit operation. They merrily blow up. Much smoke and ash when they go. Flames out the top cover are common - and much smoke.

If one output is gone (unlikely, many are expected to be dead), you still must do one of two things.

Get the M.E. upgrade, choose your metal level .

Or, get a set of outputs (a repair kit really) from M.E., if you plan to buy your own and match them, then budget two days and 40 of each number. I am not kidding here. They are that tight. You will need a jig that mounts 4 at a time and a temperature probe. Plus test gear and good DC supplies. I've done this.

hi anatech,
the problem is that one channel's fets is getting very, very hot and the other is cool, when i adjust the bias currents it holds on for a short time and then it happens once again, some times the opposite ch is getting hot, that's why i need the schematics for
sa-220if it is possible.
when is getting very hot are a lot of distortions as well.
i will try to use the sa-100 drawings, so........
best regards
williams

Hi Williams,
This amp uses a standard Vbe multiplier with an added emitter degeneration resistor. The thermal time constant is very long and the tracking is not ideal.

Remove one supply fuse and install a 1 ohm resistor. You will measure the voltage drop across this resistor. At 1/3 nominal AC input voltage, you should measure less than a 10 mV drop. Anything greater than this indicates a defective mosfet.

The bias current should be set for a reading of 0.48 to 0.52 VDC across the 1 ohm resistor. The bias will climb with temperature. You should allow at least 1/2 hour for the bias to settle down before readjusting. The entire procedure will easily require 1/2 day or more.

hi Chris,
thanks for the prompt answer,
i will do it during the weekend looks simple that means about 0.5amps as dc bias , i hope nothing is wrong with the fets.
by the way the new hexfets plastic-power from irf can do it as a replacement?
best regards
Williams

Hi Williams,
Due to the high power dissipation from each output, I would strongly recommend you stay with the originals. Never mind the horror story involved with matching them close enough. You really want to avoid having to do this.

BTW, never tighten the screws just short of damaging the sockets as stated in the manual. Follow all mounting recommendations from IR or MOT. Overtightening the parts will increase the thermal resistance of the interface and may crack the die inside the part.

Basically, don't play with the outputs unless there is no other choice.